Wideband signals are defined as high frequency or high bit rate signals such as video, bulk data or facsimile transmissions to differentiate this type of transmission from the low frequency or low bit rate signals such as standard data or voice transmissions. The earliest wideband switching systems were developed in conjunction with the Picturephone.RTM. visual telephone service developed by AT&T. The Picturephone equipment was a collection of terminal devices which used analog baseband signals to transmit black and white television signals between the calling and the called parties to provide face-to-face, full motion video communications. The switching systems used to interconnect the Picturephone terminal equipment were space division analog switching systems. These switching systems had the capacity to switch only a limited bandwidth analog signal through the analog crosspoints and the terminal equipment served by these systems were limited to transmitting a baseband analog black and white television signal.
A number of other video terminal devices were developed subsequent to the introduction of Picturephone visual telephone service which devices attempted to conserve signal bandwidth by using slow scan or video signal compression techniques. These terminal devices were successful in conserving bandwidth but the quality of the signals transmitted by this equipment was unsatisfactory to customers. As a result of these limitations, the field of wideband switching was not commercially successful. There were a number of different switching systems and terminal equipment developed, but none achieved success in the market place.
Space division switching of analog signals in business communication systems was supplanted by time division switching of digital signals. These time division switching systems first digitally encode the analog transmissions from the terminal equipment served by these switching systems and then transmit the digitally encoded signals to a selected destination over a time multiplexed bus. Time division switching systems use a common time multiplexed bus to carry the digitally encoded signals between the terminal equipment and these time division busses have a limited bandwidth. A video signal requires significant bandwidth, especially if the transmission is of a color picture. The transmission capacity of a typical time division bus in a business communication system would quickly be occupied by a very few digitally encoded color television signals. Thus, it is impractical to use time division switching and the traditional business communication system architecture to transmit video signals.
An alternative to business communication systems for carrying video signals is the cable television or CATV type of system. These systems are typically coaxial cable based broadcast systems. The brandwidth capacity of a coaxial cable is significantly greater than a typical business communication system time division bus. The coaxial cable can concurrently carry a number of color TV transmissions. These transmissions are frequency multiplexed onto the coaxial cable and broadcast to all terminals served by the coaxial cable based system.
There are several difficulties associated with a CATV broadcast system. One of these is that the coaxial cable can only carry a certain finite number of transmissions. This number determines the size of the system, that is, how many concurrent transmissions can be broadcast and how many terminals can be served by the system. Another difficulty is that all the terminal equipment must be equipped with frequency shifting modems. The modems interconnect the terminal equipment with the coaxial cable transmission media. These modems must be able to shift frequencies under control of some control signal to translate the desired signal received on the coaxial cable from the frequency at which that signal is transmitted down to the frequency that is required for the terminal equipment. These frequency agile modems are expensive and the cost of such devices is proportional to the range of frequencies which these devices must span. Therefore, in a coaxial cable system that is capable of carrying a significant number of concurrent transmissions, these modems must be able to span a wide range of frequencies and are therefore very costly devices. Another limitation or disadvantage is that the coaxial cable system does not contain any capability for security. All receivers can access all transmissions carried by the coaxial cable.